1. Field of the Invention
The present invention relates to a brushless motor driving device, and particularly to abnormality detection for AD conversion in a brushless motor driving device.
2. Description of the Related Art
A brushless motor driving device uses phase current (U-phase current, V-phase current, W-phase current) and phase voltage (U-phase voltage, V-phase voltage, W-phase voltage) of a brushless motor in order to control the brushless motor or monitor the status of the brushless motor.
The phase-current and the phase-voltage are passed through a phase-current detection circuit or a phase-voltage detecting current and then input to a microcomputer, and then they are subjected to AD (analog-to-digital) conversion in the microcomputer and used for predetermined processing for motor driving control, etc.
FIG. 12 schematically shows an AD converting circuit. Analog signals from input terminals (Ch1 to Ch4) are input to a multiplexer 61. In the multiplexer 61, a switch 65 selects any one of the input terminals (Ch1 to Ch4) at a predetermining timing, and accumulates the voltage of the input terminal thus selected (Ch1 in FIG. 12) as charges in a capacitor 62. On the basis of the charges thus accumulated, the voltage of the selected input terminal is successively compared with an AD-converted reference voltage in a voltage comparing circuit 63 to determine the voltage of the input voltage. The AD-conversion result of the voltage of the input terminal is stored in a conversion data storing register 64. The microcomputer reads out the AD-conversion result from the conversion data storing register 64 on demand and predetermined processing is carried out.
Here, when Ch1 and Ch2 of these input terminals are short-circuited (inter-pin short-circuit), the voltages of Ch1 and Ch2 are equal to each other, and thus a correct voltage value cannot be achieved.
Particularly the brushless motor driving device has a tendency that when the driving duty of a switching element 301 for driving the U-phase in the motor driving circuit 50 of FIG. 3 increases, the phase-current flowing in the U-phase (that is, AD voltage=voltage after AD-conversion) increases and the phase-voltage (that is, AD voltage) decreases as shown in FIG. 5. The same is true of the V-phase and the W-phase. Accordingly, when the U-phase current, the V-phase current, the W-phase current, the U-phase voltage, the V-phase voltage, the W-phase voltage is input to the neighboring input terminals (Ch1 to Ch6) as shown in FIG. 5, it cannot be detected that Ch1 and Ch2 or Ch5 and Ch6 are short-circuited to each other.
Furthermore, when AD conversion is carried out on the input terminal Ch2 under an open state (pin-floated state) where the input terminal Ch2 is floated from a circuit board due to failure of soldering between the input terminal Ch2 and the circuit board and thus no correct voltage is applied to the input terminal Ch2, the charges corresponding to the voltage of the input terminal Ch1 remain accumulated, so that the AD-conversion result of the voltage of the input terminal Ch1 is stored in the storage area of the input terminal Ch2 of the conversion data storing register 64. Under this state, an opening failure of the input terminal Ch2 cannot be detected.